This invention relates to superabsorbent polymers with odor control properties.
Water-absorbent polymers, also referred to as superabsorbent polymers or aqueous fluid absorbent polymers, are primarily used in personal care products which absorb body fluids, for example, baby diapers, adult incontinence products and feminine hygiene products. In such applications, superabsorbent polymer particles are incorporated into absorbent structures that contain synthetic and/or natural fiber or paper based, woven and nonwoven structures, or toughened masses of fibers, such as fluff pads. The materials used in such structures can quickly absorb aqueous fluids and distribute them throughout the whole absorbent structure. The structures, in the absence of superabsorbent polymers, have limited absorption capacity, are bulky due to the large amount of material needed to provide acceptable absorption capacity, and do not retain fluid under pressure. A means for improving the absorbency and fluid retention characteristics of such absorbent structures is to incorporate superabsorbent polymer particles which imbibe fluids to form a swollen hydrogel material.
The superabsorbent polymer particles quickly absorb fluids and retain such fluids to prevent leakage and give the absorbent structure a “dry feel” even when wetted. See U.S. Pat. No. 4,610,678 for examples of such polymers. See also U.S. Pat. Nos. 4,654,039 and Re. 32,649, which disclose a process for the preparation of superabsorbent polymers and the use of known crosslinking agents for such polymers, and also U.S. Pat. Nos. 4,295,987 and 4,303,771. A variation of the basic process is taught in GB Patent 2,119,384, which discloses a post polymerization surface crosslinking process in which the previously polymerized absorbent polymer powder is mixed with crosslinkers, preferably polyalcohols, a solvent and water, to coat the polymer surface and is heated to temperatures in the range of 90 to 300° C. to crosslink the surface. U.S. Pat. No. 5,506,324 discloses superabsorbent polymer particles comprising polymers containing carboxyl moieties which are crosslinked using C2-10 polyhydric hydrocarbons which are ethoxylated with from 2 to 8 ethylene oxide units per hydroxyl moiety of the polyhydric hydrocarbon wherein the hydroxyl moiety at the end of each ethylene oxide chain is esterified with a C2-10 unsaturated carboxylic acid or ester thereof. In a preferred embodiment, the superabsorbent polymer particles are subjected to a heat-treatment process after drying and sizing the particles.
It would be desirable, especially for the use of superabsorbent polymers in feminine hygiene products and adult incontinence products, to have a superabsorbent polymer that reduces unpleasant odors that might develop in use, particularly when contacted with bacteria infected urine. Different methods have been employed in the prior art to reduce malodor in superabsorbent polymer-containing devices.
Various odor-controlling agents are known in the prior art. Odors can be in general chemically classified as being basic, acidic and neutral. Odor-controlling agents can combat odors based on different mechanisms such as, for example, absorption, adsorption and inclusion complexation of malodor causing molecules, masking and modification of malodor causing molecules, inhibition of malodor producing microorganisms or a combination of these mechanisms.
European Patent Publication 392 608 discloses a disposable absorbent polymer product which comprises a cyclodextrin, especially β-cyclodextrin, and an active agent, for example, a perfume. WO 99/64485 also relates to superabsorbent polymers containing cyclodextrins. However, cyclodextrins are biologically degradable, and are a good nurture for microorganisms. When contacted with microorganisms, such as the bacteria in infected urine, bacteria proliferation is increased, resulting in increased malodor. Furthermore, cyclodextrins are often very fine dusty substances that are difficult to handle on a large commercial processing scale.
U.S. Pat. No. 4,385,632 is directed to an absorbent article for urine which contains a water-soluble copper salt, for example copper acetate, which impedes bacterial growth, prevents ammonia production and binds ammonia by complexation so as to prevent the occurrence of unpleasant odor. The copper ion treatment is less favorable not only due to its low efficacy even at relatively high concentrations in the case of heavy incontinence where severe urinary tract infection is present, but also due to coloring which may limit its use in hygiene articles from the aesthetic viewpoint.
U.S. Pat. No. 6,096,299 discloses an absorbent article containing an odor control material that comprises a zeolite having a particle size of more than 200 μm. The zeolite may optionally be mixed with a superabsorbent polymer and activated carbon. WO 98/20915 concerns a superabsorbent composition containing a superabsorbent polymer powder and a zeolite powder exchanged with metal cations having bactericidal properties, such as Ag, Cu and Zn ions. It is a disadvantage of zeolite materials that they are less effective in controlling odor when used in swollen superabsorbent polymer gels. It is assumed that the odor absorbing capacity, that is, the pores of the zeolite, may partially be filled by water molecules instead of volatile odor-causing molecules. Furthermore, zeolite materials are in general fine dusty substances that are difficult to handle on a large commercial scale.
Japanese Patent Publication 05179053 relates to a method for producing a water absorbent resin with good antimicrobial properties wherein the resin contains a water-insoluble inorganic phosphate compound, for example, silver sodium hydrogen zirconium phosphate (sold under the tradename Antimicrobial ALPHASAN RC 5000 by Milliken Chemicals, USA). The inorganic phosphate compound has a general formula of M1aAbM2c(PO4)d.nH2O. M1 is selected from Ag, Cu, Zn, Sn, Hg, Pb, Fe, Co, Ni, Mn, As, Sb, Bi, Ba, Cd and Cr. A is selected from alkali metal ions, alkaline earth metal ions, NH4 and H, preferably, M1 is, for example, Ag; A is, for example, Li, Na, NH4 or H; M2 is, for example, Zr, Ti or Sn. It is assumed that the M1 ions captured in the network structure of the specified phosphate compound are released as in the case of heavy metal ion exchanged zeolites. However, these inorganic phosphate compounds have drawbacks similar to those of the zeolite materials mentioned above.
WO 00/78281 discloses an anti-microbial absorbent product comprising homogeneously dispersed particles of metallic silver having a particle size in the range of 1 to 50 nm. One embodiment relates to a disposable absorbent article comprising superabsorbent polymers. However, the preparation of the silver nano-particles is complicated.
As seen above, most existing methods are incapable of sufficiently reducing malodor, or have other drawbacks. They often require treatments with malodor adsorbents, or perfume/fragrance. The use of perfume/fragrance can mask the malodor but it can be difficult to match the personal odor preference of the user. Often, the offensiveness of the combination of malodor and perfume is perceived to be more than that of the malodor alone. The various treatments in the prior art involve complicated and time-consuming process steps and are also often detrimental with regard to the absorption capacity and other properties of the superabsorbent polymer. Therefore, it would be highly desirable to provide a superabsorbent polymer with odor control properties that is unaffected in its absorbency properties. It also would be desirable to develop a simple process for preparing the superabsorbent.